The chemical composition of a star determines the orbits of gas giant planets, a new study has found. The result implies we'll be unlikely to find life around stars with high metal content, helping us focus our searches on those places likely to represent suitable hosts.
When astronomers first started finding planets outside the Solar System, the easiest to find, and therefore the bulk of early discoveries, were what we call “hot Jupiters”. These are giant planets with masses similar to Jupiter or larger orbiting so close to their star that they reach phenomenal temperatures. The cause of these planets was a mystery, and given their power to destroy otherwise habitable planets, an important one to solve in the quest for life.
It is thought hot Jupiters don't form so close to their star, instead starting further out and migrating in. The process is likely to wreak havoc on any Earth-sized planets in the habitable zone, so we can certainly be grateful Jupiter never made the trek, while wondering why.
In a paper accepted for by the Astrophysical Journal (preprint on arXiv), the Technical University of Denmark's Professor Lars Buchhave has taken advantage of recent discoveries of 16 planets in Jupiter-like orbits to see what they have in common.
Buchhave and his co-authors report that the known planets with “masses, orbits and eccentricities comparable to Jupiter (Jupiter analogues)” orbit stars with metal content similar to the Sun, while hot Jupiters are found around stars with higher metal content. When discussing star content, metal means any element heavier than helium.
Reinforcing the connection, the paper observes that more metallic stars also often host Jupiter-mass planets in their outer system in highly eccentric (i.e. elongated) orbits. Eccentric orbits are believed to be a precursor to planets migrating inwards, so it is unsurprising that even higher metal stars have gas giants that orbit far closer in than Mercury.
This still leaves the question of causality, but the authors think they have this covered as well. They note that modeling suggests metal-rich stars should spawn multiple Jupiter-mass planets; “Leading to planet-planet interactions and, hence, a prevalence of either eccentric cool Jupiters or hot Jupiters with circularized orbits.”
The paper acknowledges this isn't the whole story, since three Jupiter analogs have been found around reasonably high-metal stars. Nevertheless, the pattern strongly suggests high metal stars are bad places to seek planets capable of supporting life. Since it is also thought low metal stars will have few planets, there may be a narrow window in metal content where life is likely. Since a star's metal content is one of the easiest things about it to determine, this gives us a great clue as to where we should be looking.